Conversational query expansion methods [1]}, [2]}, [3]} generate a standalone query \(\hat{Q}_i\)  for a conversational query \(Q_i\)  by adding to \(Q_i\)  a set of query terms \(\mathcal {S}\) . The expansion terms are often selected from its dialog history \(\mathcal {H}_i\)  to optimize the retrieval result of \(\hat{Q}_i\)  as
\(\mathcal {S}^* &= \text{argmax}_{\mathcal {S} \subseteq \mathcal {H}_i} \text{Accuracy}(Q_i \cup \mathcal {S}),\) 
